1. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a system and method that provides an ABL-based modulation technique for a multicarrier communication system.
2. Description of the Related Art
In Orthogonal Frequency Division Multiplexing (OFDM) systems, a number of typically sinusoidal carriers are used for transmitting information. In legacy OFDM wireless LAN systems, all carriers have the same modulation. When a channel becomes too noisy, all carriers fall back to a simpler degree or level of modulation, thereby lowering the overall data rate that can be achieved. An Adaptive Bit Loading (ABL) modulation technique has been proposed that allows a more continuous rate fall back in order to increase throughput in colored-noise environments. ABL modulation basically provides carriers having a low signal-to-noise ratio (SNR) with a simpler modulation (or the carrier is not transmitted at all), and carriers having a high SNR with a higher complexity modulation. That is, the modulator changes the number of bits assigned to a given symbol based on the channel conditions while meeting the required target bit error rate (BER) at the receiver. Accordingly, more data can be sent when the channel conditions are good and less data will be sent when channel conditions are poor while still meeting the target BER.
ABL has been utilized in, for example, Digital Subscriber Line (DSL) systems. DSL channels, however, are typically rather benign relative to wireless channels in that the DSL environment does not change rapidly. For additional details regarding ABL, see, for example, Chow et al., “A Practical Discrete Multitone Transceiver Loading Algorithm for Data Transmission over Spectrally Shaped Channels,” IEEE Trans. Comm. Vol. 43, pp. 773-775, 1995.
One problem with ABL for OFDM, such as is used in 802.11a, in which 48 carriers and seven modulations are used and that a simplistic ABL coding would have a significant impact on throughput because the information fed back would include approximately 348 bits. The channel, however, contains much redundancy and it is unlikely that the highest SNR signals and the lowest SNR signals would alternate within the channel.
U.S. Patent Application Publication No. 2003/0072382 A1 discloses source-trellis coding for achieving data compression. Simple trellis coding is not adaptive and can be computationally complex. In particular, the trellis coding technique disclosed in U.S. Patent Application Publication No. 2003/0072382 A1 finds the “optimal” ABL constellation vector. After the optimal ABL constellation vector has been found, the ABL constellation vector is then trellis source coded. The term “ABL vector” as used herein denotes an Ncarrier symbol sequence. Each symbol represents a particular modulation on a given carrier and there are N carriers per OFDM symbol. For 802.11a, Ncarrier=48. The symbols in the ABL vector take on values from an alphabet {a1, a2, . . . , anumber of modulations}. Thus, the modulation that is to be used in j-th symbol is aj. In 802.11a, the number of modulations is 8. In actuality, ABL implementations need not be optimal and reducing complexity of the operation of determining an ABL constellation vector is critical because the complexity of the operation directly affects the complexity of the receiver.
Consequently, what is needed is a source-coded ABL-based technique for a multicarrier communication system that having a reduced computational complexity and interoperatively scales between a complex implementation and an implementation that uses a minimal form of an ABL-based multicarrier modulation technique or uses no ABL-based modulation technique.